As known in the art, automated working machines, such as machine tools and industrial robots, apply forces to workpieces and are themselves subjected to external forces because of the manner in which these machines operate. In this case, it is necessary for the working machines to detect external forces and moments applied to the machines and to perform control corresponding to the detected external forces and moments. In order to perform the control, corresponding to the detected external forces and moments, with a high degree of precision, it is required to accurately detect the external forces and moments.
In view of this situation, various types of force sensors have been proposed to date. Generally, the force sensors can be classified, according to the detection scheme employed, into elastic-type force sensors and equilibrium-type force sensors. The elastic-type force sensors measure a force on the basis of an amount of deformation proportional to the external force, while the equilibrium-type force sensors measure a force by balancing it with a known force.
Also known are force sensors whose structure is based on the principle that a plurality of strain resistance elements are provided in parts of a strain-generating body that is elastically deformable in response to an external force applied thereto. When an external force is applied to the strain-generating body of the force sensor, electrical signals corresponding to the degree of deformation (stress) of the strain-generating body are output from the plurality of strain resistance elements. Forces that have two or more components and are applied to the strain-generating body can be detected on the basis of these electrical signals, and a stress produced in the force sensor is calculated on the basis of the electrical signals.
Among examples of the conventionally-known elastic-type force sensors are six-axis force sensors, each of which includes a plurality of strain resistance elements provided in parts of a strain-generating body. The six-axis force sensors divide an external force applied thereto into stress components (i.e., forces Fx, Fy, Fz) in respective axial direction of three axes (i.e., X-axis, Y-axis and Z-axis) of an orthogonal coordinate system and into torque components (i.e., moments Mx, My, Mz) about the respective axis directions, and it detects the external force as six-axis components.
Generally, in the field of multi-axis force sensors, there would be encountered the problem of interference from other axes (i.e., inter-axis interference problem) that prevents individual components (i.e., forces and moments) of an external force, applied to the strain-generating body, from being accurately separated from one another or resolved with good precision. The inter-axis interference problem can not be ignored when putting a multi-axis force sensor to practical use.
As a technique for solving the inter-axis interference problem, the inventors of the present invention etc. proposed a six-axis force sensor, having a novel construction, in Japanese Patent Laid-Open Publication No. 2003-207405 (hereinafter “Patent Literature 1”). This proposed six-axis force sensor can provide a solution to the problem of interference from other axes (i.e., inter-axis interference problem) that prevents individual components (i.e., forces and moments) of an external force, applied to the strain-generating body, form being accurately separated from one another or resolved with good precision. In the proposed six-axis force sensor, a plurality of strain resistance elements are integrally assembled in a predetermined arrangement or layout pattern in parts of a strain-generating body on a semiconductor substrate by using semiconductor manufacturing processing. The proposed six-axis force sensor is formed using the semiconductor substrate of a substantially square planar shape, which includes: a supporting part located in an outer peripheral portion of the semiconductor substrate, an operating part located in a central portion of the semiconductor substrate and having a substantially square shape, and connecting parts connecting the four side of the square operating part and corresponding portions of the supporting part. The strain resistance elements are provided on boundary areas between the individual sides of the square operating part and the connecting parts. The proposed six-axis force sensor is arranged to solve the “inter-axis interference” problem through an improvement in the configuration of parts of the strain-generating body and optimization of the layout pattern of the plurality of strain resistance elements.
Further, in Japanese Patent Application Laid-open Publication No. 2003-254843 (hereinafter “Patent Literature 2”), a six-axis force sensor is proposed where a structure having an external-force buffering function is provided, in addition to the construction of the six-axis force sensor disclosed in Patent Literature 1, so that an external force, having been attenuated by the buffering structure is applied to the operating part. Thus, the six-axis force sensor disclosed in Patent Literature 2 can achieve an enlarged range of detectable external forces.
However, in each of the six-axis force sensor chips (semiconductor substrates) disclosed in Patent Literature 1 and Patent Literature 2 identified above, stress tends to concentrate in the operating part due to bending or twisting caused by axial force application, because the operating part is located in a central portion of the force sensor chip; particularly, such stress concentrates if the operating part has a relatively small area. If the operating part has a relatively large area, on the other hand, load bearing performance can be enhanced; however, there is a tradeoff between the enhanced load bearing performance and miniaturization of the operating part. As a consequence, the disclosed six-axis force sensors are considerably limited in design freedom depending on a state of stress occurring in the operating part.